``Nobody really believed the results before. There was so much fraud,'' says Guadalupe Colin, a gregarious university student here. ``I think we're going to see real democracy, a real election for the first time,'' she says during a break in a training course for Mexican citizens volunteering to be among an estimated 35,000 registered electoral observers.

``These will be the most important, most contested, most transparent, most democratic, most observed, most judged, and most criticized elections in Mexican history,'' concurs Peter Smith, a Mexico expert at the University of California at San Diego.

The scrutiny at home and abroad combined with a plethora of recent reforms to the electoral process means that the world's longest-ruling political party is at a milestone, if not a tombstone. Actually, the PRI is leading in the polls. But win or lose, the PRI is looking at the unknown. For the first time in its 65-year history, the PRI's widely acknowledged ability to ``influence'' the outcome of elections has been sharply curbed.

``The rules of the game are changing. The PRI regime is arriving at its historical limit,'' says Jose Antonio Crespo, a Mexican political scientist.

The change is creating a potent blend of tension and excitement that hangs in the air like the turgid, dirty-gray rain clouds that cover the August skies here.

And the elections are yet another in a string of explosive events that have jarred Mexicans since January. In the southern Mexican state of Chiapas, an armed Mayan uprising erupted on New Year's Day, shaking the government with its demands for greater democracy.

The uprising was followed by the assassination of the original PRI candidate, Luis Donaldo Colosio; a spate of kidnappings; street battles between narcotraffickers; and a run on the Mexican peso.

The expectation of further unrest - depending as much on how clean the elections are as on who wins - keeps the tension high. And that expectation has contributed to unusually close US attention to the process, by everyone from the White House to human rights activists.

US gets involved

Last week, the US Congress passed a resolution of ``support'' for Mexico's democratic progress and ``encouraged'' its southern neighbor to continue its efforts at ensuring that clean elections be held.

Separately, the budget-strapped US Agency for International Development sprung loose $550,000 to send US electoral observers to Mexico, who will be among some 1,000 foreigners (mostly from the US) here to oversee the voting.

And, for the first time, a group of seven United States and European banks have financed a nationwide poll to get a handle for themselves on who is likely to win.

``The PRI has been winning elections any way it can for decades. Why is the US interested now?'' asks Jorge Alcocer, a political newsmagazine editor.

The short and simplistic answer: the North American Free Trade Agreement (NAFTA) and the risk of riots or revolt.

A shared 2,000-mile open border has always kept Mexico highlighted on the US foreign-policy map. It is a strategic source of oil and a partner (generally reliable) in the battle against illegal trafficking of narcotics. (Up to half of all cocaine sold in the US transits Mexico).

But the US has taken Mexico's political stability for granted during 65 years without a change in ruling party. Now, the potential for instability, from the rebels in Chiapas to disgruntled losers who may cry fraud, causes many to worry about increased Mexican migration flows into the US. Illegal migration is already a major political and economic issue in US border states, particularly California.

NAFTA has made US officials even more attentive because of the growing interdependence of the two nations. ``If we don't have peaceful transition of power here, that will substantially affect the US,'' says Rafael Fernandez de Castro, professor of international relations at the Mexican Autonomous Technological Institute here.

In May, Mexico quietly replaced Japan as the second-biggest importer of US goods, after Canada. During the last five years, trade has surged dramatically with everything from Apple computers to Haagen-Dazs ice cream becoming available. Since NAFTA went into effect on Jan. 1, US exports to Mexico have soared, up 15.7 percent in the first quarter.

And trade with Mexico is burgeoning beyond the traditional US border states. For example, Mexico is Connecticut's No. 1 export market.

Last year, it sold more than $300 million worth of products here, up 18.1 percent over the previous year. Not bad considering the Mexican economy has been in a slump for the past year. Sixteen US states have opened trade offices in Mexico City in the last three years.

The threat of instability

But if post-electoral violence erupts, the flood of US goods and investment could drop off, endangering Mexican and US jobs. Most Mexican and US analysts expect the PRI to win the election, but that does not ensure peace. The PAN and PRD have promised civil unrest if fraud occurs.

This threat of instability has Mexicans discussing several post-election scenarios based on Sunday's vote.

* A Second Round. The PRI wins by a wide margin - 55 percent or more - a number the opposition parties will find hard to accept. The PRD or the PAN (or both) cry fraud and mount massive street demonstrations. The present administration has negotiated three times to replace an officially elected PRI candidate in state and local elections after such protests. If this happens, a peace pact installing an acceptable interim president could result.

* The Chiapas Factor. A more radical variation of a Second Round. The Mayan Indian rebels who staged this year's armed uprising say that if the PRI wins fraudulently, they will attack again. This could prompt other radical groups to join in guerrilla-type civil conflict. The government is taking the threat seriously, searching for weapons caches in poor, rural states and setting up extra police patrols on highways in Guerrero State, where drug runners and bandits are active.

* An Opposition Victory. The PAN or PRD wins. A PAN win is likely to be more palatable to the PRI than a PRD win. But either way, local PRI supporters may object and seek a second round.

* An Acceptable PRI Victory. The PRI wins the presidency, receiving less than 55 percent of the votes, but loses the majority in Congress. The PAN and PRD do well. The PAN endorses the elections as clean. The PRD protests, but lacks sufficient support to get a second round.

PRI insiders are hoping for this last scenario. ``Zedillo will win by a small margin. You'll see a real change, a peaceful, democratic change in the Chamber of Deputies [the lower house] where the PRI will lose the majority,'' says a Mexican shipping magnate. ``That will bring an end to presidencialismo [unchecked presidential power], which is something we all want.''

Indeed, be they PRI or opposition party supporters, it appears most Mexicans want more than a simulated democracy. What happens on Aug. 21, and the Mexican reaction to that vote, will be closely watched by much of North America.